The crankshaft is spun up in a machine with a strobe light and some very sensitive switches. If there's a measurable amount of shake, the light flashes, "freezing" the motion at a particular position. A meter indicates how far out of balance the part is. When a "heavy" spot on the shaft is found, a big drill press built into the machine drills a little weight off one of the counterweights to minmize the vibration. If the counterweight is too light, sometimes material is welded onto the shaft. For really radical weight changes, a hole is drilled through a counterweight, and a piece of "Mallory Metal",heavier than lead, is riveted into the hole. By adding or removing weight at the right places, the vibration of the shaft when it turns can be minimized or eliminated.
Jerry
